It has been known that a (2S,4S) substance of 4-hydroxy-4-(3-indolylmethyl)-2-aminoglutaric acid (hereinafter, it will be sometimes called “monatin”) represented by the following formula (3) is contained in bark of root of Schlerochitom ilicifolius which is a plant naturally grown in the Northern Transvaal of South Africa, has a sweet taste which is several hundred times as sweet as sucrose and is an amino acid derivative useful as a sweetener (refer to Patent Document 1: Japanese Patent Laid-Open No. 64/025,757).

Monatin has asymmetric carbonatoms at 2- and 4-positions and has the following four optical isomers.

With regard to a process for producing monatin, there have been many reports (refer to Non-Patent Document 1: Tetrahedron Letters, 2001, Vol. 42, No. 39, pages 6793 to 6796; Non-Patent Document 2: Organic Letters, 2000, Vol. 2, No. 19, pages 2967 to 2970; Non-Patent Document 3: Synthetic Communication, 1994, Vol. 24, No. 22, pages 3197 to 3211; Patent Document 2: U.S. Pat. No. 5,994,559; Patent Document 3: Japanese Patent Laid-Open No. 2002/060,382; etc.) and, although there are examples where several processes for producing optically active monatin are investigated, they need very many steps for the production and are not able to be said to be industrially suitable processes for production.
On the other hand, the present applicant has recently found and reported for a process in which a monatin precursor is synthesized from indole-3-pyruvic acid and, via steps where a diastereomer salt with a specific optically active amine is formed followed by subjecting to an optical resolution, a specific optically active monatin is finally produced (refer to Patent Document 4: WO 03/059,865) When (2R,4R) monatin is taken as an example, the process is able to be represented by the following scheme.

As compared with the conventional processes, the above producing process comprises less steps, is able to produce an optically active monatin in an efficient manner and is a producing process which is suitable in industry as well. However, an optical isomer concerning 2-position which is out of the object is separated to the mother liquor side and, if it is isomerized, for example, to convert to an aimed optically active monatin, the efficiency is further enhanced.
The present applicant has also found and reported that, among the four optical isomers of monatin, monatins where steric configuration at 2-position is R or, in other words, (2R,4R) monatin and (2R,4S) monatin are far better in terms of intensity as a sweetener than monatins where steric configuration at 2-position is S (refer to Patent Document 5: WO 03/045,914) Therefore, it is convenient in using monatin as a sweetener if 2-position of (2S,4S) monatin or (2S,4R) monatin having lower strength of sweetness is isomerized whereby each of them is able to be efficiently induced to (2R,4S) monatin or (2R,4R) monatin, respectively.
With regard to a method for racemization of optically active substance of amino acid or the like, there have been known, for example, a method where treatment is conducted under strongly acidic or strongly alkaline condition or at high temperature and a method where racemization is conducted under a relatively mild condition in the presence of an aldehyde (refer to Patent Document 6: Japanese Patent Laid-Open No. 57/123,150; and Patent Document 7: Japanese Patent Laid-Open No. 58/167,562) but, in those methods, the maximum yield is about 50% since isomerization ratio is converged to 1:1 whereby they are unable to be said to be efficient racemization method. A method of isomerization and separation in which aimed optical isomer is produced in a high yield by combining with a specific optically active substance has been also known (refer to Non-Patent Document 4: Tetrahedron, 1997, Vol. 53, No. 28, pages 9417 to 9476; Non-Patent Document 5: Tetrahedron Asymmetry, 2002, Vol. 13, pages 2649 to 2652) but enormous trials-and-errors are necessary for finding such a combination.    [Patent Document 1]
Japanese Patent Laid-Open No. 64/025,757    [Patent Document 2]
U.S. Pat. No. 5,994,559    [Patent Document 3]
Japanese Patent Laid-Open No. 2002/060,382    [Patent Document 4]
WO 03/059,865    [Patent Document 5]
WO 04/045,914    [Patent Document 6]
Japanese Patent Laid-Open No. 57/123,150    [Patent Document 7]
Japanese Patent Laid-Open No. 58/167,562    [Non-Patent Document 1]
Tetrahedron Letters, 2001, Vol. 42, No. 39, pages 6793 to 6796    [Non-Patent Document 2]
Organic Letters, 2000, Vol. 2, No. 19, pages 2967 to 2970    [Non-Patent Document 3]
Synthetic Communication, 1994, Vol. 24, No. 22, pages 3197 to 3211    [Non-Patent Document 4]
Tetrahedron, 1997, Vol. 53, No. 28, pages 9417 to 9476    [Non-Patent Document 5]
Tetrahedron Asymmetry, 2002, Vol. 13, pages 2649 to 2652